This invention relates to a pipeline cable deployment apparatus and method. That is to say, it is concerned with deploying cable inside a pipeline. It is particularly suitable for the deployment of a communication cable, for example, an optical fibre or an electrical cable, containing an optical or electrical conductor, but could be adapted for use in deploying other kinds of cable.
The invention has been developed for use in the oil and gas extraction and transportation industries, and in water and gas utility pipelines, for the deployment of cable into an existing pipeline. Suitable pipelines include transmission pipelines and observation wells. The invention is not primarily intended for use in a down-well environment, but such an application is nevertheless feasible.
The purpose of deploying cable along a pipeline may be to enhance communications from one end of the pipe to the other, or to install sensors into the pipeline for intelligence gathering or for diagnostic purposes. The sensors may be particular instruments or may be integral with the cable itself. For example, as is known in the art, a fibre optic cable can function as a distributed temperature measuring device, in which the optical properties of the cable itself provide data from throughout the length of the cable by the reflection of light introduced from one end of the cable back to detectors at the same end.
In most instances, a cable is fed into a pipeline by pulling it off a reel or spool at a convenient point at one end of a pipeline or along the pipeline, and drawing the cable into and along the pipeline by using a cable towing device such as a pipeline pig, or a drogue, or by relying on friction between the cable itself and fluid flowing through the pipeline to draw the cable into and along the pipeline for the required distance.
However, the distances involved may be a matter of kilometers, and the cable needs to be sufficiently strong that lengths of cable of that order can be drawn into the pipeline under tension. Strength in turn implies weight, which may in turn require more strength. Another problem is the problem of abrasion as the cable is drawn through the pipeline for thousands of meters.
The present invention is made in response to the observation that the foregoing deployment methods rely on a supply of cable from outside the pipeline, giving rise to problems in feeding the cable into the pipeline. It is accordingly proposed instead to take a fundamentally different approach, namely to take the cable supply through the pipeline and pay it out as it goes. The major benefit of such an approach is that the deployed cable is stationary with respect to the pipeline, thereby eliminating a major source of impact and abrasion damage, and furthermore the cable does not have to be made strong enough to withstand the tension required to pull thousands of meters of its own weight through a long tube.
More specifically, a method of deploying a cable, typically a communication cable, in a pipeline comprises driving a pipeline pig forwardly along the pipeline away from one restrained end of a coil of said cable extending out of a rearwardly directed exit from an enclosure carried by the pig.
In another aspect of the invention, pipeline cable deployment apparatus comprises a pipeline pig having, in use, forward and rearward ends, and carrying an enclosure containing a coil of cable, the enclosure having an exit for the cable rearwardly of the pig.
The pipeline pig may be self-propelled. A self-propelled pig may also be called a crawler. The pig may be drawn behind a tractor. However, although the invention does embrace the use of crawlers and of towed pigs, in preferred embodiments of the invention the pig is driven by fluid pressure in the pipeline.
The pig may include sensors, such as a television camera or acoustic sensors, from which real time signals can be carried by a communication cable to remote instrumentation. It will be apparent that other forms of cable than communication cable can be deployed in accordance with the invention without departing from the fundamental concepts. In the case of a crawler, or a pig drawn by a tractor, power supply cable may be included for delivering power to a drive motor of the crawler or tractor.
Pipeline pigs are well known in the art and any appropriate pig may be used. A typical pig has a body and laterally extending means for positioning the pig in the pipeline. In the case of a fluid driven pig, the laterally extending means may be flanges or skirts extending outwardly from the pig body towards the pipeline walls. Normally, of course, pipelines tend to be cylindrical, but there is no reason in principle why other shapes should not be compatible with the invention. A suitable pig may have lateral flanges at its forward and rearward ends to stabilise it and guide it in the pipeline and allow it to be driven forward by the pipeline fluid.
The enclosure for the coil of cable may suitably be carried within the pig, on the outside of the pig, or behind the pig. It may conveniently be a fixed or removable cassette which can be single use or reloaded with a fresh coil of cable for a cable deployment run. The enclosure or cassette may be located within the pig body, or behind the rearmost flange or skirt on the rearward end of the body. The cable should be freely deployable from the exit of the enclosure well clear of any possible impediments to its smooth release. If mounted on the rearward end of the pig, the cassette or enclosure may be flexibly connected to the pig, to articulate the assembly for use in a curved pipeline.
The coil of cable may take any convenient form. Preferably it remains static with respect to the enclosure. Cable may be paid out from an outer or inner surface of the coil. Although the coil may be formed on a rotating spool or the like, this has a disadvantage in that the angular momentum of the rotating coil and spool will tend to pay out excess cable if the pig is suddenly slowed in the pipeline. Conical or cylindrical coils of cable may be suitable.
A typical coil is made up of multiple layers of cylindrical windings. These can be formed by winding cable on to a collapsible mandrel, building up as many cylindrical layers as are required to accommodate the desired cable length. The mandrel can then be collapsed and removed leaving a hollow coil which can be placed in a cylindrical cassette, one end of the cable from the inside of the coil being drawn out through the exit of the cassette in order to be fastened to an anchorage before deployment of the cable through a pipeline by loading the cassette on a pig and driving the pig through the pipeline.
Desirably, the coiled cable is coated with a weak adhesive, sufficient to bind the turns of the coil together unless tension is applied to the cable to part them. Suitable binders for this purpose include waxes and varnishes, such as paraffin wax, or silicone resins, especially for use with fibre optic communication cable. The cable itself should not be so thick that it takes a permanent deformation when coiled, or cannot readily exit the enclosure. The coiling process must not apply stresses to the cable beyond its elastic limits. The cable should be windable, pliable, and uncoilable with a tendency to avoid kinking, snagging on itself or hockling (the formation of tight spiral loops in twisted cables when lengthwise tension is relaxed).
To at least partially reduce these problems, the cable may be given a back twist, by up to one turn per coil of cable, when the coil is being formed, so that it can be more cleanly uncoiled.
Suitable materials for the cables are any that serve the intended purpose, particularly of providing communication means, and tolerate the pipeline environment. Any compatible fluid may be used to drive the deploying pig through the pipeline. A typical fibre optic cable may consist of a silica glass core, a silicone sheath to the core, and an outer coating of PTFE to impart abrasion resistance and to resist high temperatures. Cables may be reinforced, for example by Kevlar(trademark) fibre braiding, to add tensile strength, especially for steeply inclined or vertical pipelines.